This invention generally relates to an improved process for reacting N-substituted N-(phosphonomethyl)glycines (sometimes referred to as “N-substituted glyphosates”), salts of N-substituted N-(phosphonomethyl)glycines, and esters of N-substituted N-(phosphonomethyl)glycines to form N-(phosphonomethyl)glycine (sometimes referred to as “glyphosate”), salts of N-(phosphonomethyl)glycine, and esters of N-(phosphonomethyl)glycine via a noble-metal catalyzed oxidation reaction. This invention is particularly directed to such reactions using N-substituted N-(phosphonomethyl)glycines, salts of N-substituted N-(phosphonomethyl)glycines, and esters of N-substituted N-(phosphonomethyl)glycines which have a single N-carboxymethyl functionality.
N-(phosphonomethyl)glycine is described by Franz in U.S. Pat. No. 3,799,758, and has the following formula:
N-(phosphonomethyl)glycine and its salts conveniently are applied as a post-emergent herbicide in an aqueous formulation. It is a highly effective and commercially important broad-spectrum herbicide useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
Various methods for the preparation of N-(phosphonomethyl)glycine from N-substituted N-(phosphonomethyl)glycines are known in the art. For example, in U.S. Pat. No. 3,956,370, Parry et al. teach that N-benzylglycine may be phosphonomethylated to N-benzyl N-(phosphonomethyl)glycine, and then reacted with hydrobromic or hydroiodic acid to cleave the benzyl group and thereby produce N-(phosphonomethyl)glycine. In U.S. Pat. No. 3,927,080, Gaertner teaches that N-t-butylglycine may be phosphonomethylated to form N-t-butyl N-(phosphonomethyl)glycine, and then converted into N-(phosphonomethyl)glycine via acid hydrolysis. N-(phosphonomethyl)glycine also may be produced from N-benzyl N-(phosphonomethyl)glycine via hydrogenolysis, as described, for example, in European Patent Application No. 55,695. A separate discussion directed to producing N-(phosphonomethyl)glycine from N-benzyl N-(phosphonomethyl)glycine via hydrogenolysis may be found in Maier, L., Phosphorus, Sulfur and Silicon, 61, 65–7 (1991). These processes are problematic in that they produce undesirable byproducts such as isobutylene and toluene which create difficulties due to their potential toxicities. Moreover, acid hydrolysis and hydrogenation of N-substituted N-(phosphonomethyl)glycines have been reported only for hydrocarbyl groups such as tertiary butyl and benzyl groups which are generally known to be susceptible to such reactions; there has not been reported a general method for dealkylation of N-substituted N-(phosphonomethyl)glycines.
Other methods for the preparation of N-(phosphonomethyl)glycine include those directed to oxidatively cleaving N-(phosphonomethyl)iminodiacetic acid (sometimes referred to as “PMIDA”):
PMIDA may be synthesized, for example, from phosphorus trichloride, formaldehyde, and an aqueous solution of the disodium salt of iminodiacetic acid, as described by Gentilcore in U.S. Pat. No. 4,775,498:
This reaction is complicated by the necessity of removing sodium chloride from the PMIDA product. Sodium chloride has low solubility in the presence of HCl due to the common ion effect, and both iminodiacetic acid and PMIDA are insoluble in HCl and in water under neutral conditions. Thus, salt separation requires that the NaCl be dissolved after the reaction forming PMIDA is complete. This is done by neutralizing the HCl with a base, and then adding water to ensure that all the NaCl dissolves. This large volume of water leads to significant losses of PMIDA during recovery, and increases the volume of waste.
Various methods for converting PMIDA into N-(phosphonomethyl)glycine are well known in the art. These include:    1. Heterogeneous catalytic oxidation. This method is discussed, for example by Franz in U.S. Pat. No. 3,950,402. A separate discussion may be found in Balthazor et al., U.S. Pat. No. 4,654,429.    2. Homogeneous catalytic oxidation. This method is described, for example, in Riley et al., J. Amer. Chem. Soc. 113, 3371–78 (1991). A separate discussion may be found in Riley et al., Inorg. Chem., 30, 4191–97 (1991).    3. Electrochemical oxidation using carbon electrodes. This method is described, for example, by Frazier et al. in U.S. Pat. No. 3,835,000.Such methods oxidatively remove one of the two N-carboxymethyl groups from PMIDA. Generally, such oxidative decarboxylations rely on a one-electron oxidation of PMIDA accompanied by loss of carbon dioxide to form a carbon based radical. The radical is then oxidized to N-(phosphonomethyl)glycine in a subsequent one-electron step. These reactions are summarized as follows:
Oxidative decarboxylations, in general, are well known in the art, particularly for electrochemical oxidations (also known as the Kolbe reaction). The Kolbe reaction is particularly facile with carbon electrodes. See, e.g., S. Torii and H. Tanaka, Organic Electrochemistry, 535–80 (H. Lund and M. M. Baizer eds., Marcel Dekker, 3rd ed. 1991).
The methods used to oxidize PMIDA to N-(phosphonomethyl)glycine have not been reported to be useful for preparing N-(phosphonomethyl)glycine from N-substituted N-(phosphonomethyl)glycines having only one N-carboxymethyl group, i.e., where R′ in the following formula is a functionality other than a carboxymethyl:
If R′ is other than a carboxymethyl, removal of R′ typically requires a single, two-electron oxidation of the N-substituted N-(phosphonomethyl)glycine, rather than two successive one-electron oxidations.